Catalytic converters are devices for converting noxious exhaust components into less toxic agents. Catalytic converters conventionally comprise a substrate structure, a catalyst support layer bonded thereto, and one or more catalytically-active agents bonded to the catalyst support layer.
Substrate structures house the catalyst and provide conduits for the exhaust fumes which are to be chemically scrubbed. They have conventionally been fabricated from ceramic material, although the use of thin metal substrates has gained considerable commercial acceptability in recent years owing to the ability of metal substrates to be made in larger cross-sections (as would be needed for the treatment of large gas flows), the ability to manufacture metal substrates which are thinner than ceramic substrates, and the easier fabrication of structures made from metal.
The catalyst support layer is used as a support for the catalyst, and therefore, must have the property of both bonding to the substrate and the catalyst itself. Typically, such catalyst support layer is formed by repeatedly dipping the substrate into a slurry which contains particles of the catalyst support material and drying and/or calcining the resulting coated product. Catalyst support layers manufactured by such a "dipping" technique are customarily referred to as "washcoat." Catalytic converter catalyst support layers are generally comprised of such substances as activated alumina (Al.sub.2 O.sub.3) (U.S. Pat. No. 4,601,999), silicas, and mixed oxide powders of silica, vanadia and titania (U.S. Pat. No. 5,272,125). In thin metal converters, the catalyst support layer is preferably placed upon the substrate after the substrate has been exposed to heat in an oxidizing atmosphere. Heating of the metallic substrate in an oxidizing atmosphere results in an adherent self-healing oxide diffusion barrier which prevents further oxidation and thus protects the metal core. The barrier also prevents base metal in the core from diffusing into the catalyst support coating.
Several catalysts may be affixed to the catalyst support layer which is bound to the metal substrate forming a catalyst member. The catalyst(s) being employed in any given converter are dependent upon the chemical conversion desired and the temperature range over which the converter will operate. For example, noble metals such as platinum and palladium are often used in the treatment of auto exhaust to promote oxidation of unburned or partially oxidized hydrocarbons and to promote the reduction of nitrogen oxides because of their durability at high temperatures. Metal oxide catalysts, such as those formed with metals of Groups V and VI of the Periodic Table, are frequently employed for vapor phase catalytic oxidation of organic compounds. Catalysts are conventionally adhered to the catalyst support layer by means of liquid-carrier impregnation.
Catalytic converters may be fabricated with either monofunctional or polyfunctional catalyst members. Monofunctional catalyst members are capable of catalyzing only one type of chemical reaction, such as oxidation. On the other hand, polyfunctional catalyst members (ie., a metal substrate coated with catalyst support material bound to a plurality of catalysts) may catalyze a plurality of chemical reactions. Of the polyfunctional catalyst members, the "three-way conversion" ("TWC") catalyst member is frequently employed in the art. TWC catalyst members are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. TWC catalyst members may comprise either a single or multiple deposition layer of catalyst-impregnated catalyst support material.
U.S. Pat. No. 4,294,726 (the "726 patent") describes an example of a polyfunctional catalyst member comprising a single layer of catalyst-catalyst support material. The '726 patent discloses a TWC catalyst composition containing platinum and rhodium obtained by impregnating a gamma alumina carrier material with an aqueous solution of cerium, zirconium and iron salts or mixing the alumina with oxides of, respectively, cerium, zirconium and iron, and then calcining the material at 500 to 700.degree. C. in air after which the material is impregnated with an aqueous solution of a salt of platinum and a salt of rhodium, dried and subsequently treated in a hydrogen-containing gas at a temperature of 250-650.degree. C. The alumina may be thermally followed by impregnating the treated carrier material with aqueous salts of platinum and rhodium and then calcining the impregnated material.
U.S. Pat. No. 5,057,483, on the other hand, discloses a TWC catalyst member comprised of a catalytic material disposed in two discrete coats on a carrier. The first coat includes a stabilized alumina support on which a first platinum catalytic component is dispersed, and bulk ceria, and may also include bulk iron oxide, a metal oxide (such as bulk nickel oxide) which is effective for the suppression of hydrogen sulfide emissions, and one or both of baria or zirconia dispersed throughout the first coat as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed (e.g. co-precipitated) rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second activated alumina support having a second platinum catalytic component dispersed thereon. The second coat may also include a second rhodium catalytic component, and optionally, a third platinum catalytic component, dispersed as a activated alumina support.
Likewise, International Application No. PCT/US94/07235 discloses a polyfunctional catalyst member having the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides in a gas stream over wide temperature ranges. The bottom layer is disclosed to comprise an oxygen storage component, e.g. ceria, in intimate contact with a platinum group metal. The bottom layer composition provides sufficient oxygen storage capacity to enhance CO oxidation and nitrogen oxide reduction at temperatures above about 500.degree. C. The top layer comprises a palladium catalytic component free from intimate contact with an oxygen storage component. The top layer provides sufficient catalytic activity with respect to hydrocarbon and nitrogen oxide conversion during the initial heating of the catalytic converter and at an operating temperature below 500.degree. C. Performance of the catalyst is enhanced in both layers by the use of an alkaline earth metal as a stabilizer, a rare earth metal component as a promoter of the reaction, and a zirconium component to enhance both the stabilizer and promoter. Layering of the bottom layer catalyst composition with the top layer catalyst composition is said to both simplify production of the catalyst substrate and to enhance its efficacy. Exhaust gaseous emissions first encounter the top layer composition where the platinum group metal acts to catalyze the reduction of nitrogen oxides to nitrogen and the oxidation of hydrocarbons. Upon passing through the top layer, the exhaust gas then contacts the bottom layer, where the platinum group metal is in intimate contact with an oxygen storage component thereby enhancing oxidation and reduction reactions at high temperatures.
As disclosed in International Application No. PCT/US94/07235 and U.S. Pat. No. 4,294,726, multi-layer TWC catalyst members have several significant advantages over single layer polyfunctional catalyst members. The primary advantage is that different catalyst/adjuvant compositions can be segregated to such an extent that the exhaust fumes can first react with the outer catalyst layer and then with the inner catalyst layer. Unintended chemical reactivity is significantly reduced by such segregation.
For reasons set forth above, many manufacturers of catalytic converters have begun using metals to form catalytic converter substrate structures. Further, in recent years, many manufacturers have come to favor metal substrates over ceramic substrates because of the widespread incorporation of heaters into catalytic converters to enhance catalysis at relatively low "start-up" temperatures. Metal substrates enhance the efficacy of such heaters owing to their propensity to heat quicker than ceramic materials.
Traditionally, metal catalytic members have been manufactured by dipping the metal substrate into a catalyst support material slurry, drying the resulting coated metal and then impregnating the coated metal substrate with catalyst by means of a catalyst slurry. This traditional method, however, suffers from a number of disadvantages. First the method is time consuming in that it requires that the catalyst support layer be dried prior to application of the catalyst. The method also requires that two slurries be prepared--a catalyst support layer slurry, and a catalyst slurry--increasing the expense involved in the coating process, and the possibility of introduction of error. Additionally, due to the nature of metals, especially the flexibility inherent in thin metal strips, catalyst support material frequently does not adhere well to the metal substrate. Adherence of the catalyst support layer is especially problematic in corrugated metal foils, as corrugations have the tendency to resist uniform coating. Corrugated substrate structures are frequently employed in catalytic converters today since they increase surface area for increased reactivity between the exhaust gases and the catalyst. Catalyst support material applied to corrugated converter structures by "dipping" often preferentially collects in the negative radius of curvature areas of the structure (valleys or crevices). This preferential collection results in a catalyst support layer of non-uniform thickness.
In order to avoid the problems associated with "dipping" metal substrates into slurries, it has been suggested that electrophoretic deposition be employed to deposit catalyst support materials onto the metal substrate.
While several attempts have been made to use electrophoretic deposition in the fabrication of metal substrates coated with catalyst support materials, it is not believed that heretofore electrophoretic deposition has been used to produce a commercially acceptable catalyst support layer on such substrates. In co-pending U.S. patent application Ser. No. 08/600,585 however, a system employing electrophoretic deposition which provides for an acceptable metallic-substrate based catalytic converter is described. Such system encompasses contacting a metal foil with an aqueous slurry of catalyst support material and catalyst, placing an electrode in contact with said slurry, applying an electric field between the metal foil and the electrode whereby the foil becomes a cathode and the electrode an anode, maintaining the electric field for a time sufficient to cause deposition of at least some of the catalyst support material and catalyst onto the metal foil, removing the coated foil from the electric field, and drying the coated foil.
While electrophoretic deposition has the potential to significantly improve the production of metal catalytic members, conventional electrophoretic deposition, as well as that described in co-pending U.S. patent application Ser. No. 08/600,788 filed Feb. 13, 1991 now U.S Pat. No. 5,795,456, does not lend itself to the formation of multi-layer catalyst members. The latter is a result of the electrophoretic process itself, wherein the metal is coated with minimally electrically-conductive material. The coating inhibits further deposition of material onto the metal after a certain coating thickness is achieved. That is, the first catalyst-impregnated support material layer, a significant part of which is generally ceramic support, forms a dielectric barrier upon application onto the metal substrate which prohibits further deposition of material. As presently known in the art, the only viable method for applying a second catalyst-impregnated catalyst support material layer to an electrophoretically-deposed catalyst member is to "dip" the dried member into a second catalyst support layer slurry, and then to impregnate the same with catalyst.
A method of electrophoretically depositing a plurality of layers onto a metal substrate would significantly improve the efficiency of production of multi-layer polyfunctional metal catalyst members by reducing the laborious steps of: removing the electrophoretically-deposited catalyst member from its support, drying the catalyst member, contacting it with a slurry of catalyst support material, drying, and then impregnating with a second (third, fourth, etc.) catalyst system. Such a method would further improve the uniformity of any second (third, fourth, etc.) layer applied to the first (or subsequent layer), significantly reducing waste product.
It recently has been discovered that multi-layer catalyst members can be formed without performing such laborious steps. The latter is accomplished by means of maintaining electrical interaction between the metal substrate which has been coated, the power source, and the electrodes disposed in the electrophoretic deposition ("EPD") cell containing the slurry of catalyst support material and catalyst which are to form a layer on top of the first layer. Maintenance of electrical interaction may be by means of masking certain sections of the foil prior to the first layer deposition, and then unmasking these sections after deposition to permit an electrical contact between the metal substrate and the power source to allow a second layer deposition. More preferably, however, electrical interaction is maintained between the foil, the power source and the electrodes by point penetration through the first layer, thereby maintaining electrical contact between the foil and the negative terminal of a power supply. Such method permits the foil to remain acting as the cathode to the bath electrodes (anodes). The latter method further provides for the continuous deposition of multi-layer nonidentical catalysts onto the metal substrate, without the need (as in the first method) from stopping the production line and removing the masking.